1. Field of the Invention
The present invention is in the field of fiber optic acoustic sensor arrays wherein light is propagated in the arrays and the effects of acoustic signals on the light returning from the arrays are analyzed to determine the characteristics of the acoustic signals.
2. Description of the Related Art
Fiber optic based acoustic sensors are promising alternatives to conventional electronic sensors. Included among their advantages are a high sensitivity, large dynamic range, light weight, and compact size. The ability to easily multiplex a large number of fiber optic sensors onto common busses also makes fiber optic sensors attractive for large-scale arrays. The recent successful incorporation of multiple small-gain erbium doped fiber amplifiers (EDFAs) into a fiber optic sensor array to increase the number of sensors that can be supported by a single fiber pair has made large-scale fiber optic sensor arrays even more competitive.
For acoustic detection, the fiber optic sensor of choice has been the Mach-Zehnder interferometric sensor. In any interferometric sensor, phase modulation is mapped into an intensity modulation through a raised cosine function. Because of this nonlinear transfer function, a sinusoidal phase modulation will generate higher order harmonics. An interferometer biased at quadrature (interfering beams .pi./2 out of phase) has a maximized response at the first order harmonic and a minimized response at the second order harmonic. For this reason, quadrature is the preferred bias point. As the bias point drifts away from quadrature (for example, due to external temperature changes), the response at the first order harmonic decreases and the response at the second order harmonic increases. When the interferometer is biased at 0 or .pi. out of phase, the first order harmonic disappears completely. This decreased response at the first order harmonic (resulting from the bias points away from quadrature) is referred to as signal fading.
Because Mach-Zehnder interferometric sensors have an unstable bias point, they are especially susceptible to the signal fading problem just mentioned. In order to overcome signal fading, a demodulation of the returned signal is required. The typical demodulation technique is the Phase-Generated Carrier (PGC) scheme, which requires a path-mismatched Mach-Zehnder interferometric sensor. (See, for example, Anthony Dandridge, et al., Multiplexing of Interferometric Sensors Using Phase Carrier Techniques, Journal of Lightwave Technology, Vol. LT-5, No. 7, July 1987, pp. 947-952.) This path imbalance also causes the conversion of laser phase noise to intensity noise, which limits the performance of the Mach-Zehnder interferometric sensor arrays at low frequencies and places stringent requirements on the linewidth of the source. This narrow linewidth requirement has slowed the development of amplified Mach-Zehnder interferometric sensor arrays at 1.55 .mu.m.
The Sagnac interferometer has found widespread use in the fiber optic gyroscopes. (See, for example, B. Culshaw, et al., Fibre optic gyroscopes, Journal of Physics E (Scientific Instruments), Vol. 16, No. 1, 1983, pp. 5-15.) It has been proposed that the Sagnac interferometer could be used to detect acoustic waves. (See, for example, E. Udd, Fiber-optic acoustic sensor based on the Sagnac interferometer, Proceedings of the SPIE-The International Society for Optical Engineering, Vol. 425, 1983, pp. 90-91; Kjell Krakenes, et al., Sagnac interferometer for underwater sound detection: noise properties, OPTICS LETTERS, Vol. 14, No. 20, Oct. 15, 1989, pp. 1152-1145; and Sverre Knudsen, et al., An Ultrasonic Fiber-Optic Hydrophone Incorporating a Push-Pull Transducer in a Sagnac Interferometer, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 12, No. 9, September 1994, pp. 1696-1700.) Because of its common-path design, the Sagnac interferometer is reciprocal and therefore has a stable bias point, which eliminates signal fading and prevents the conversion of source phase noise into intensity noise. Therefore, the Sagnac interferometer is immune to the phase noise which limits the Mach-Zehnder interferometric sensors at low frequencies.